Electrical
Circuits &Machines
UNIT III: DC
Motors
MCQ’S
-Prof. Tushar V.
Kafare
BSIOTR Wagholi
1.
Laminations of core are generally made of
(a) case
iron
(b) carbon
(c) silicon
steel
(d)
stainless steel
Ans: c
2. Which of
the following could be lamina-proximately the thickness of laminations of a
D.C.
machine ?
(a) 0.005 mm
(b) 0.05 mm
(c) 0.5 m
(d) 5 m
Ans: c
3. The
armature of D.C. generator is laminated to
(a) reduce
the bulk
(b) provide
the bulk
(c) insulate
the core
(d) reduce
eddy current loss
Ans: d
4. The
resistance of armature winding depends on
(a) length
of conductor
(b)
cross-sectional area of the conductor
(c) number
of conductors
(d) all of
the above
Ans: d
5. The field
coils of D.C. generator are usually made of
(a) mica
(b) copper
(c) cast
iron
(d) carbon
Ans: b
6. The
commutator segments are connected to the armature conductors by means of
(a) copper
lugs
(b)
resistance wires
(c)
insulation pads
(d) brazing
Ans: a
7. In a
commutator
(a) copper
is harder than mica
(b) mica and
copper are equally hard
(c) mica is
harder than copper
(d) none of
the above
Ans: c
8. In D.C.
generators the pole shoes are fastened to the pole core by
(a) rivets
(b) counter
sunk screws
(c) brazing
(d) welding
Ans: b
9. According
to Fleming's right-hand rule for finding the direction of induced e.m.f., when
middle
finger points in the direction of induced e.m.f., forefinger will point in the
direction
of
(a) motion
of conductor
(b) lines of
force
(c) either
of the above
(d) none of
the above
Ans: b
10.
Fleming's right-hand rule regarding direction of induced e.m.f., correlates
(a) magnetic
flux, direction of current flow and resultant force
(b) magnetic
flux, direction of motion and the direction of e.m.f. induced
(c) magnetic
field strength, induced voltage and current
(d) magnetic
flux, direction of force and direction of motion of conductor
Ans: b
11. While
applying Fleming's right-hand rule to And the direction of induced e.m.f., the
thumb points
towards
(a) direction
of induced e.m.f.
(b)
direction of flux
(c)
direction of motion of the conductor if forefinger points in the direction of
generated
e.m.f.
(d)
direction of motion of conductor, if forefinger points along the lines of flux
Ans: d
12. The
bearings used to support the rotor shafts are generally
(a) ball
bearings
(b) bush
bearings
(c) magnetic
bearmgs
(d) needle
bearings
Ans: a
13. In D.C.
generators, the cause of rapid brush wear may be
(a) severe
sparking
(b) rough
commutator surface
(c)
imperfect contact
(d) any of
the above
Ans: d
14. In lap
winding, the number of brushes is always
(a) double
the number of poles
(b) same as
the number of poles
(c) half the
number of poles
(d) two
Ans: b
15. For a
D.C. generator when the number of poles and the number of armature conductors
is fixed,
then which winding will give the higher e.m.f. ?
(a) Lap
winding
(b) Wave
winding
(c) Either
of (a) and (b) above
(d) Depends
on other features of design
Ans: b
16. In a
four-pole D.C. machine
(a) all the
four poles are north poles
(b)
alternate poles are north and south
(c) all the
four poles are south poles
(d) two
north poles follow two south poles
Ans: b
17. Copper
brushes in D.C. machine are used
(a) where
low voltage and high currents are involved
(b) where
high voltage and small cur-rents are involved
(c) in both
of the above cases
(d) in none
of the above cases
Ans: a
18. A
separately excited generator as compared to a self-excited generator
(a) is
amenable to better voltage con-trol
(b) is more
stable
(c) has
exciting current independent of load current
(d) has all
above features
Ans: d
19. In case
of D.C. machines, mechanical losses are primary function of
(a) current
(b) voltage
(c) speed
(d) none of
above
Ans: c
20. Iron
losses in a D.C. machine are independent of variations in
(a) speed
(b) load
(c) voltage
(d) speed
and voltage
Ans: b
21. In D.C.
generators, current to the external circuit from armature is given through
(a)
commutator
(b) solid
connection
(c) slip
rings
(d) none of
above
Ans: a
23. Brushes
of D.C. machines are made of
(a) carbon
(b) soft
copper
(c) hard
copper
(d) all of
above
Ans: a
24. If B is
the flux density, I the length of conductor and v the velocity of conductor,
then
induced
e.m.f. is given by
(a)Blv
(b)Blv2
(c)Bl2v
(d)Bl2v2
Ans: a
25. In case
of a 4-pole D.C. generator provided with a two layer lap winding with sixteen
coils, the
pole pitch will be
(a) 4
(b) 8
(c) 16
(d) 32
Ans: b
26. The
material for commutator brushes is generally
(a) mica
(b) copper
(c) cast
iron
(d) carbon
Ans: d
27. The
insulating material used between the commutator segments is normally
(a) graphite
(b) paper
(c) mica
(d)
insulating varnish
Ans: c
28. In D.C.
generators, the brushes on commutator remain in contact with conductors
which
(a) lie
under south pole
(b) lie
under north pole
(c) lie
under interpolar region
(d) are
farthest from the poles
Ans: c
29. If
brushes of a D.C. generator are moved in order to bring these brushes in
magnetic
neutral
axis, there will be
(a)
demagnetisation only
(b) cross
magnetisation as well as magnetisation
(c)
crossmagnetisation as well as demagnetising
(d) cross
magnetisation only
Ans: c
30. Armature
reaction of an unsaturated D.C. machine is
(a)
crossmagnetising
(b)
demagnetising
(c)
magnetising
(d) none of
above
Ans: a
31. D.C.
generators are connected to the busbars or disconnected from them only under
the
floating
condition
(a) to avoid
sudden loading of the primemover
(b) to avoid
mechanicaljerk to the shaft
(c) to avoid
burning of switch contacts
(d) all above
Ans: d
32. Eddy
currents are induced in the pole shoes of a D.C. machine due to
(a)
oscillating magnetic field
(b)
pulsating magnetic flux
(c) relative
rotation between field and armature
(d) all
above
Ans: c
33. In a
D.C. machine, short-circuited field coil will result in
(a) odour of
barning insulation
(b)
unbalanced magnetic pull producing vibrations
(c)
reduction of generated voltage for which excitation has to be increased to
maintain the
voltage
(d) all
above
Ans:
34.
Equilizer rings are required in case armature is
(a) wave
wound
(b) lap
wound
(c) delta
wound
(d) duplex
wound
Ans: b
35. Welding
generator will have
(a) lap
winding
(b) wave
winding
(c) delta
winding
(d) duplex
wave winding
Ans: a
36. In case
of D.C. machine winding, number of commutator segments is equal to
(a) number
of armature coils
(b) number
of armature coil sides
(c) number
of armature conductors
(d) number
of armature turns
Ans: a
37. For a
D.C. machines laboratory following type of D.C. supply will be suitable
(a) rotary
converter
(b) mercury
are rectifier
(c)
induction motor D.C. generator set
(d) synchronous motor D.C. generator set
Ans: c
38. The
function of pole shoes in the case of D.C. machine is
(a) to
reduce the reluctance of the magnetic path
(b) to
spread out the flux to achieve uniform flux density
(c) to
support the field coil
(d) to
discharge all the above functions
Ans: d
39. In the
case of lap winding resultant pitch is
(a)
multiplication of front and back pitches
(b) division
of front pitch by back pitch
(c) sum of
front and back pitches
(d)
difference of front and back pitches
Ans: d
40. A D.C.
welding generator has
(a) lap
winding
(b) wave
moving
(c) duplex
winding
(d) any of
the above
Ans: a
41. Which of
the following statement about D.C. generators is false ?
(a)
Compensating winding in a D.C. machine helps in commutation
(b) In a D.
C. generator interpoles winding is connected in series with the armature
winding
(c) Back
pitch and front pitch are both odd and approximately equal to the pole pitch
(d) Equilizing
bus bars are used with parallel running of D.C. shunt generators
Ans: d
42. The
demagnetising component of armature reaction in a D.C. generator
(a) reduces
generator e.m.f.
(b)
increases armature speed
(c) reduces
interpoles flux density
(d) results
in sparking trouble
Ans: a
43. Magnetic
field in a D.C. generator is produced by
(a)
electromagnets
(b)
permanent magnets
(c) both (a)
and (b)
(d) none of
the above
Ans: a
44. The
number of brushes in a commutator depends on
(a) speed of
armature
(b) type of
winding
(c) voltage
(d) amount
of current to be collected
Ans: d
45.
Compensating windings are used in D.C. generators
(a) mainly
to reduce the eddy currents by providing local short-circuits
(b) to
provide path for the circulation of cooling air
(c) to
neutralise the cross-magnetising effect of the armature reaction
(d) none of
the above
Ans: c
46. Which of
the following components of a D.C, generator plays vital role for providing
direct
current of a D.C. generator ?
(a) Dummy
coils
(b) Commutator
(c) Eye bolt
(d)
Equilizer rings
Ans: b
47. In a
D.C. generator the ripples in the direct e.m.f. generated are reduced by
(a) using
conductor of annealed copper
(b) using
commutator with large number of segments
(c) using
carbon brushes of superior quality
(d) using
equiliser rings
Ans: c
48. In D.C.
generators, lap winding is used for
(a) high
voltage, high current
(b) low
voltage, high current
(c) high
voltage, low current
(d) low
voltage, low current
Ans: b
49. Two
generators A and B have 6-poles each. Generator A has wave wound armature
while
generator B has lap wound armature. The ratio of the induced e.m.f. is
generator A
and B will
be
(a) 2 : 3
(b) 3 : 1
(c) 3 : 2
(d) 1 : 3
Ans: b
50. The
voltage drop for which of the following types of brush can be expected to be
least ?
(a) Graphite
brushes
(b) Carbon
brushes
(c) Metal
graphite brushes
(d) None of
the above
Ans: c
51. The
e.m.f. generated by a shunt wound D.C. generator isE. Now while pole flux
remains
constant, if
the speed of the generator is doubled, the e.m.f. generated will be
(a) E/2
(b) 2E
(c) slightly
less than E
(d) E
Ans: b
52. In a
D.C. generator the actual flux distribution depends upon
(a) size of
air gap
(b) shape of
the pole shoe
(c)
clearance between tips of the ad¬jacent pole shoes
(d) all of
the above
Ans:
53. The
armature core of a D.C. generator is usually made of
(a) silicon
steel
(b) copper
(c)
non-ferrous material
(d)
cast-iron
Ans: a
9.63.
Satisfactory commutation of D.C. machines requires
(a) brushes
should be of proper grade and size
(b) brushes
should smoothly run in the holders
(c) smooth,
concentric commutator properly undercut
(d) all of
the above
Ans: d
54. Open
circuited armature coil of a D.C. machine is
(a)
identified by the scarring of the commutator segment to which open circuited
coil is
connected
(b)
indicated by a spark completely around the commutator
(c) both (a)
and (b)
(d) none of
the above
Ans: c
55. In a
D.C. machine, fractional pitch winding is used
(a) to
increase the generated voltage
(b) to
reduce sparking
(c) to save
the copper because of shorter end connections
(d) due to
(b) and (c) above
Ans:
56. For the
parallel operation of two or more D.C. compound generators, we should ensure
that
(a) voltage
of the incoming generator should be same as that of bus bar
(b) polarity
of incoming generator should be same as that of bus bar
(c) all the
series fields should be run in parallel by means of equilizer connection
(d) series
fields of all generators should be either on positive side or negative side of
the
armature
(e) all
conditions mentioned above should be satisfied
Ans: d
57. D.C.
series generator is used
(a) to
supply traction load
(b) to
supply industrial load at constant voltage
(c) voltage
at the toad end of the feeder
(d) for none
of the above purpose
Ans: c
58.
Following D.C. generator will be in a position to build up without any residual
magnetism in
the poles
(a) series
generator
(b) shunt
generator
(c) compound
generator
(d)
self-excited generator
Ans: d
59.
Interpole flux should be sufficient to
(a)
neutralise the commutating self induced e.m.f.
(b)
neutralise the armature reaction flux
(c)
neutralise both the armature reaction flux as well as commutating e.m.f. induced
in the
coil
(d) perform none of the above functions
Ans: c
60. D.C.
generator generally preferred for charging automobile batteries is
(a) series
generator
(b) shunt
generator
(c) long
shunt compound generator
(d) any
of'the above
Ans: c
61. In a D.C.
generator the number of mechanical degrees and electrical degrees will be the
same when
(a) r.p.m.
is more than 300
(b) r.p.m.
is less than 300
(c) number
of poles is 4
(d) number
of poles is 2
Ans: d
62.
Permeance is the reciprocal of
(a) flux
density
(b)
reluctance
(c)
ampere-turns
(d)
resistance
Ans: b
63. In D.C.
generators the polarity of the interpoles
(a) is the
same as that of the main pole ahead
(b) is the
same as that of the immediately preceding pole
(c) is
opposite to that of the main pole ahead
(d) is
neutral as these poles do not play part in generating e.m.f.
Ans: a
64. The
e.m.f. generated in a D.C. generator is directly proportional to
(a)
flux/pole
(b) speed of
armature
(c) number
of poles
(d) all of
the above
Ans: b
65. In a
D.C. generator the magnetic neutral axis coincides with the geometrical neutral
axis, when
(a) there is
no load on|he generator
(b) the
generator runs on full load
(c) the generator runs on overload
(d) the
generator runs on designed speed
Ans: a
66. In a
D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f.
in the
coil is
neutralised by all of the following except
(a)
interpoles
(b) dummy
coils
(c)
compensating winding
(d) shifting
of axis of brushes
Ans: b
67. In D.C.
generators on no-load, the air gap flux distribution in space is
(a)
sinusoidal
(b)
triangular
(c)
pulsating
(d) flat
topped
Ans: d
68. A shunt
generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed
increases to
1200 r.p.m., the generated e.m.f. will be nearly
(a) 150 V
(b) 175 V
(c) 240 V
(d) 290 V
Ans: c
69. The
purpose of providing dummy coils in a generator is
(a) to
reduce eddy current losses
(b) to
enhance flux density
(c) to
amplify voltage
(d) to
provide mechanical balance for the rotor
Ans: d
70. In a
shunt generator the voltage build up is generally restricted by
(a) speed
limitation
(b) armature
heating
(c)
insulation restrictions
(d)
saturation of iron
Ans:
71. If a
D.C. generator fails to build up the probable cause could not be
(a) imperfect
brush contact
(b) field resistance less than the critical
resistance
(c) no
residual magnetism in the generator
(d) faulty
shunt connections tending to reduce the residual magnetism
Ans: b
72. Flashing
the field of D.C. generator means
(a)
neutralising residual magnetism
(b) creating
residual magnetism by a D.C. source
(c) making
the magnetic losses of forces parallel
(d)
increasing flux density by adding extra turns of windings on poles
Ans: b
73. The
e.m.f. induced in the armature of a shunt generator is 600 V. The armature
resistance
is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be
(a) 640 V
(b) 620 V
(c) 600 V
(d) 580 V
Ans: d
74. In a
D.C. generator the critical resistance refers to the resistance of
(a) brushes
(b) field
(c) armature
(d) load
Ans: b
75. To
achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so
as
to bring
them
(a) just
ahead of magnetic neutral axis
(b) in
magnetic neutral axis
(c) just
behind the magnetic neutral axis
Ans: a
76. Armature
coil is short circuited by brushes when it lies
(a) along
neutral axis
(b) along
field axis
(c) in any
of the above positions
(d) in none
of the above positions
Ans: a
77. A
cumulatively compounded long shunt generator when operating as a motor would be
(a)
cumulatively compounded long shunt
(b) differentially compounded long shunt
(c)
cumulatively compounded short shunt
(d)
differentially compounded short shunt
Ans: b
78. To avoid
formation of grooves in the commutator of a D.C. machine
(a) the
brushes of opposite polarity should track each other
(b) the
brushes of same polarity should track each other
(c) brush
position has no effect on the commutator grooving
Ans: a
79. The
following constitute short-circuit in the armature winding.
(a) Insulation
failure between two com-mutator bars
(6)
Insulation failure between two turns of a coil
(c) Two of
more turns of the same coil getting grounded
(d) All of
the above
Ans: d
80. The
rapid wear of brushes takes place due to
(a) abrasion
from dust
(b) excessive
spring pressure
(c) rough
commutator bars
(d) high
mica insulation between com-mutation bars
(e) all of
the above factors
Ans: e
81. Number
of tappings for each equilizer ring is equal to
(a) number
of pole pairs
(b) number
of poles
(c) number
of parallel paths
(d) number
of commutator segments
Ans: a
82. A D.C.
generator can be considered as
(a)
rectifier
(b)
primemover
(c) rotating
amplifier
(d) power
pump
Ans: c
83. In any
rotating machine that part which houses the conductors and in which e.m.f.
induced is
to be utilised is called
(a) rotor
(b) stator
(c) field
(d) armature
Ans: d
84. In a
D.C. machine stray loss is the sum of
(a) total
copper loss and mechanical loss
(b) armature
copper loss and iron loss
(c) shunt
field copper loss and mechanical loss
(d) iron
loss and mechanical loss
Ans: d
85. Lap
winding is composed of
(a) any even
number of conductors
(b) any odd
number of conductors
(c) that
even number which is exact multiple of poles + 2
(d) that
even number which is exact multiple of poles
Ans: a
86. In a
D.C. generator in case the resistance of the field winding is increased, then
output
voltage will
(a) increase
(b) decrease
(c) remain
unaffected
(d)
fluctuate heavily
Ans: b
87. An
exciter for a turbo generator is a
(a)
separately excited generator
(b) shunt
generator
(c) series
generator
(d) compound
generator
Ans: b
88. In case
of a flat compounded generator
(a) voltage
generated is less than the rated voltage
(b)
generated voltage is proportional to the load on the generator
(c) voltage
remains constant irrespec¬tive of the load
(d) speed
varies in proportion to the load on the generator
Ans: c
89. Which of
the following generator will have negligible terminal voltage while running on
no-load ?
(a) Series generator
(b) Shunt
generator
(c) Compound
generator
(d)
Separately excited generator
Ans: a
90. Which of
the following D.C. generators will be in a position to build up without any
residual
magnetism in the poles ?
(a) Series
generator
(b) Shunt
generator
(c) Compound
generator
(d) None of
the above
Ans: d
91. In over
compounded generator, full load terminal voltage is
(a) almost
zero
(b) less
than noload terminal voltage
(c) more
than noload terminal voltage
(d) equal to
no-load terminal voltage
Ans: c
92. In a
level compounded D.C. generator, full load terminal voltage is
(a)
negligibly low
(b) equal to
no-load terminal voltage
(c) more
than no-load terminal voltage
(d) less
than no-load terminal voltage
Ans: b
93. The
terminal voltage of a D.C. shunt generator drops on load because of all of the
following
reasons except
(a) armature
reaction
(b) armature
resistance drop
(c) field
weakening due to armature reaction and armature
(d)
commutation
Ans: d
94. In a
D.C. generator
(a) external
resistance = internal char-acteristic - armature reaction
(b) internal
characteristic = magnetisation characteristic - ohmic drop
(c) external
characteristic = magnetisation characteristic - ohmic drop - armature reaction
(d)
magnetisation characteristic = external characteristic
Ans: c
95. A sinusoidal
voltage of 5 Hz is applied to the field of a shunt generator. The armature
voltage wave
(a) will be
zero
(b) will be
of 5 Hz
(c)
willbeof5xiVHz
(d) will be
of v Hz 5
Ans: b
96. A 220 V
D.C. generator is run at full speed without any excitation. The open circuit
voltage will
be
(a) zero
(b) about 2
V
(c) about 50
V
(d) 220 V
Ans: b
97. In a
separately excited generator supplying rated load the armature reaction ,
(a) is
always present
(b) is
always absent
(c) may be
sometimes present
(d) none of
the above
Ans: a
98. If
residual magnetism is present in a D.C. generator, the induced e.m.f. at zero
speed will
be
(a) zero
(b) small
(c) the same
as rated voltage
(d) high
Ans: a
99. Armature
reaction in a generator results in
(a)
demagnetisation of leading pole tip and magnetisation of trailing pole tip
(b)
demagnetisation of trailing pole tip and magnetisation of leading pole tip
(c)
damagnetising the centre of all poles
(d)
magnetising the centre of all poles
Ans: a
100.
Following energized winding of a D.C. machine should not be opened as it would
produce high
inductive voltage which may be dangerous to personnel and may cause its
own
insulation failure.
(a) Series
field
(b)
Compensating field
(c) Inter pole field
(d) Shunt
field
Ans: d
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